Tosser is a British insult for a person you think is stupid, useless or unpleasant. "He's such a tosser." "Some tosser parked across my gate." You must know what is underneath it. Tosser comes from a rude slang meaning of "toss" that refers to masturbating — so tosser literally calls someone a masturbator, in the same way that the stronger British insult "wanker" does. Native speakers do not usually think about that meaning when they say it; it has worn down into a general word for an idiot. But it IS there, and that is why the word is rude rather than merely unkind. On the British scale of insults it sits in the middle: rougher than "idiot" or "twit", noticeably softer than "wanker". You will hear it in pubs, in football crowds, in British TV dramas, from friends teasing each other, and from strangers who are genuinely angry. It is almost always used about men. It is British and Irish, and to some extent Australian. Americans mostly do not use it and may not recognise it at all. Grammar is simple: countable noun, plural tossers, used with "a" or "such a" — "a complete tosser", "an absolute tosser". I am telling you all this because you will meet the word, not because I want you to use it. Knowing why an insult is rude is part of knowing English properly — and knowing where a word sits on the scale is what keeps you from causing more offence than you meant.

混蛋、白痴（英式粗俗罵人的話）。源自「打手槍」的粗話，所以算髒話等級，比 idiot 重、比 wanker 輕，通常用來罵男生。看得懂就好，自己少用。